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— the value $B_{\varphi}$, which characterizes the glow of the light source in a given direction. B. element $d S$ surface that glows in either direction determined by the ratio of intensity $d l$ of this element in the direction of spread to the area of the projection element on a plane perpendicular to this direction: $$ B_{\varphi}=\frac{d l}{d S \cos \varphi} $$ where $\varphi$ — the angle between the normal to the element $d S$ and this area. Unit B. in SI — candela per square meter (cd/m2).